Various techniques have been proposed to sharpen the appearance of video when displayed. The need for an effective and robust sharpening filter will be particularly apparent as higher definition video begins to become a household standard and users wish to view older, lower definition recordings on higher definition monitors.
One known technique for enhancing the sharpness of video is to apply a high-frequency boosting spatial filter to the video signals to thus enhance the edges in the video. However, this technique has proven less than satisfactory for the reason that linear filters having a high-boost characteristic also exhibit ringing in their step response. This ringing will manifest as regions of luminance or color variation near to the edges of an object when placed against a background of substantially different luminance or color.
Various nonlinear filters have also been proposed for video sharpening, which for example compare the luminance values within a region and determine statistical properties Such as the minimum, maximum, average and median luminance values, from which a new value for pixels within the region can be derived. However, despite the efforts that have been applied to developing a nonlinear sharpening filter, known filters continue to display unwanted artifacts that make those filters less than completely desirable for all applications.